RESPIRATION 213 



In Frogs and Reptiles the oxygenated blood coming from the 

 lungs and that coming from the other tissues of the body mingle 

 to some extent in the heart, and this mixed blood is then sup- 

 plied to the tissues of the body. In Birds and Mammals again, 

 through the complete separation of the respiratory and systemic 

 circulation, all blood passes alternately through the lungs and 

 the body tissues. 



483. Oxygen is taken up by the blood as air is absorbed by 

 water, but in most animals, excepting Insects, there is a sub- 

 stance present in the blood which has a special affinity for oxy- 

 gen. In some cases, especially among invertebrates, this 

 substance forms part of the blood plasma; in others, including 

 all Vertebrates, it resides in certain cells floating in the blood, 

 the red blood corpuscles. In either case it gives the character- 

 istic color to the blood. In some invertebrates, the earthworm, 

 for example, and all Vertebrates, the substance is red and con- 

 tains iron. In other cases, some Crustacea and some Mollusca, 

 the substance is blue and contains copper. The first is called 

 haemoglobin, the latter haemocyanin. There are also some 

 others, more rare. These substances have an affinity for 

 oxygen, so that the blood is enabled to carry more oxygen than 

 it otherwise could. In passing through the respiratory organs 

 the oxygen carriers become charged with oxygen and assume a 

 brighter color. In passing through the tissues where oxygen is 

 needed the haemoglobin, or haemocyanin, again assume a darker 

 color because of the loss of oxygen to the tissues. The red 

 corpuscles originate in the red marrow of the bones. They are 

 short lived and disintegrate in the liver and form the red and 

 green pigments of the bile. 



METABOLISM 



484. In green plants, the protoplasm takes up inorganic 

 substances, such as water, carbon-dioxide, nitrates and other 

 mineral salts containing sulphur, phosphorus, iron, calcium, 



